I ve been reading the forums and some people are saying that destruction magic is weak in late game.

Is that true? I m doing an altmer Mage with destruction as the main tree and the other schools of magic for support.

Any advice?

no, its actually really good late game, but suffers a bit before you get the master spells. the impact perk saves it though.

edit: if you have very high magicka (what you should as high elf) plus destruction enchantments, its always good.

#1 depends on difficulty you're playing

#2: Depends if you're taking enchanting.

Adept (normal/default) or lower it's fine.

Enchanting is simply ridiculous for any build. For Destro mages you can enchant gear that gives you FREE destruction casting... so you can spam dual-cast staggering 200 point incinerates until the cows come home. Enchanting is pretty much required for Expert/Master destro play, because it just takes waaay too many casts to kill.... so it's still viable, but you will need conjuration or illusion for crowd control as you can only stun lock 1-2 enemies with impact and fights will take longer then most fighter/sneak archer builds... but still playable... and can be darn hard until you get up to 100 enchanting as well.